Crude oil is an important feedstock for the petrochemical industry
and the dominant source of energy driving the world economy. But at the
current rate of production and consumption the known reserves of oil
will be able to meet the ever-growing demand for energy for no longer
than 50 years. Oil shale, a fine-grained sedimentary rock, contains a
proportionally high amount of organic material called kerogen, and a
little bitumen, which can be converted to oil by thermal degradation.
Among alternative energy sources, oil shale appears to be particularly
attractive because of its large deposits distributed throughout the
world. The most abundant oil shale reserves are located in the United
States. Significant oil shale resources are also deposited in other
countries, such as China, Russia, Brazil, Morocco, Jordan, Australia,
Estonia and Canada.

In recent years, due to the ever-increasing demand for crude oil
and rise of its price, more attention has been paid to oil shale
resources in China, with a view to producing shale oil [1-2].

China possesses abundant resources of oil shale, its reserves being
about 31.57 billion tons. Oil shale is mainly distributed in Huadian and
Nongan of Jilin province, Fushun of Liaoning province and Maoming of
Guangdong province, all China. Retorting for producing shale oil and
burning for generating power are the main modes of its utilization
[3-4]. At present, China employs a Fushun-type retort for the commercial
retorting of lump oil shale. This retort is a vertical cylindrical
vessel, incorporating an upper section for oil shale drying and
pyrolysis, and a lower section for shale char gasification and
combustion. The heat required for pyrolysis is provided from two
sources: i.e. the hot gas produced from the gasification and combustion
of shale char with an air blast, and the hot recycle pyrolysis gas from
a recuperator entering the middle part of the retort. The Fushun-type
retort is suitable for retorting only lean oil shale, whose pyrolysis
gas is unable to supply sufficient heat for retorting. The daily
capacity of the retort is merely 100 tons of oil shale, and its shale
oil yield is but 65-70% vs. Fischer Assay [5-8].

Compared with the Fushun retort, the SJ-type pilot-scale retort
enables higher oil yield to be obtained and also its daily capacity is
higher. This means that oil shale resources are utilized in a more
efficient way, which eventually leads to higher total profit [9]. This
paper describes the basic principle and features of the pyrolysis
process carried out in the SJ-type pilot-scale retort. It also considers
the behaviour of YJ oil shale during pyrolysis in the retort. The
SJ-type pilot retort's capacity is 24 tons of oil shale per day,
making one tenth of the commercial size.

2. Features of the SJ-type retort

The process carried out in the SJ-type retort represents an
aboveground retorting method for obtaining shale oil from oil shale. It
takes its name from the developer, the Sanjiang Coal Chemical Inc. (SJ
Company), which is located in Shenmu, Shaanxi province, northwestern
China. In 2000, SJ Company started to develop an appropriate technology
for processing bituminous coal in Shenmu. With the improvement of the
rectangular gas combustion retort for oil shale processing in Maoming
[3], SJ Company developed an SJ-type retort [4].

It should be noted that the gas combustion retort is suitable for
processing relatively rich oil shale, whose pyrolysis gas can provide
sufficient heat required for the pyrolysis of oil shale.

The SJ retort uses a technology, which consists, as an advantage,
in its uniform gas distribution system, and in pushing the coke to and
cooling it at the bottom of the retort. The specially designed tar
condensing facility includes a first condenser, a Venturi tube, a second
condenser, a swirling tray tower, and an electrostatic precipitator.

The SJ retort has further advantages, such as high thermal
efficiency, high oil yield (80% of Fischer Assay), simple design, high
operational flexibility, as well as low investment costs. The retort
consists of the upper drying section, the middle pyrolysis section and
the lower coke cooling section. The mined coal is transported to a
crusher where it is ground to the lump size of 12-100 mm. Then the
material is transported on a belt to the top of the retort. Coal is fed
to the retort where it is subjected to low-temperature drying and
high-temperature retorting. The circulating retorting gas, introduced to
the middle part of the retort, is used to supply heat for retorting.
Thereafter the organic matter is released from coal as oil vapor and
retorting gas. In the next stage, coke is cooled at the bottom of the
retort.

100,000 tons of coal is processed in each SJ retort per year. The
process generates 60,000 tons of char and about 10,000 tons of coal tar.
The commercial SJ retort, with a daily capacity of 250 tons of coal, is
a rectangular vertical brick-lined steel vessel, with a height of 7176
mm and cross-section of 2600 x 4028 mm. The total residence time of coal
in the retort is about 13 hours.

An SJ-type pilot-scale retort was built by SJ Company in 2006 to
investigate the suitability of the SJ retort for processing oil shale
and lignite as well. In cooperation with SJ Company, the Yaojie Coal
Company of Gansu province, China, attempted to use the SJ retort to
process YJ oil shale. For testing oil shale samples from Jinhe (JH) and
Haikuang (HK) basins of Yaojie county were used.

The SJ pilot-scale retort is a vertical rectangular vessel. Its
height is 7176 mm and cross section 1500 x 1180 mm. Its daily capacity
is 24 tons of oil shale, which is one tenth of the commercial volume.
The condensation and recovery systems of the SJ pilot retort are similar
to those of the commercial device.

3. Process scheme used in the SJ-type gas combustion retort

The retorting process carried out in the SJ-type retort is
classified as an internal gas combustion method. Figure 1 shows the flow
diagram of the retort. Raw oil shale with a particle size of 8-60 mm is
fed to the top of the retort. In the upper section of the retort, oil
shale is dried and heated by the ascending hot gases. Heat comes from
the combustion of the recycle retorting gas. The temperature of the
exiting retorting gas is about 600 [degrees]C. The hot gas and oil vapor
move directly from the bottom to the top of the retort and then exit
through the top. The pyrolysates are shale oil, gas, water and spent
shale. After being cooled in the lower part of the retort, spent shale
is removed from the retort using a rotating water dish. Unlike the
process used in the Fushun retort, spent shale is not burnt in the lower
part of the retort [10]. Shale oil is obtained using a condensation
unit. Most of the pyrolysis gas is recirculated to the retort.

[FIGURE 1 OMITTED]

The operating parameters of the SJ pilot retort for oil shale
processing are as follows:

The volatiles content of YJ oil shale is 10-60%, that of fixed
carbon about 10%, and ash, 30-90%. The results of Fischer Assay are
presented in Table 1.

5. Pilot test results

5.1. Material balance

The material balance for oil shale retorting in the pilot retort is
shown in Table 2.

5.2. Properties of shale oil

The properties of the shale oil obtained are presented in Tables
3-5.

5.3. Calorific value of shale char

The calorific value of semicoke is given in Table 6.

5.4. Compositional analysis of the retorting gas

The composition of the retorting gas for HJ and HK oil shales is
presented in Tables 7 and 8, respectively.

From Tables 7 and 8 it is seen that the average calorific value of
the retorting gas for JH oil shale is 3930 kJ/[m.sup.3] (941
kcal/[m.sup.3]) and for HK oil shale, 5020 kJ/[m.sup.3] (1200
kcal/[m.sup.3]). The hydrogen sulfide content of the gases is very high,
which is unfavorable from an environmental point of view and the
compound needs to be removed before utilization [11].

6. Conclusions

1. The oil content of JH and HK oil shales is 15.11 and 10.86%,
respectively, indicating that the Yaojie oil shale is a valuable
resource for production of shale oil.

2. The shale oil yield during the pilot retorting in the SJ retort
is about 85% vs. Fisher Assay, in the Fushun retort, about 65%.

3. The SJ retort operated smoothly and without failure during the
Yaojie oil shale retorting. The heat needed to retort oil shale can be
provided by the self-produced retorting gas. Besides, the surplus gas
can be used for power generation.

4. The calorific value of semicoke is 1600-1900 kcal/kg. After
mixing it with oil shale less than 8 mm in particle size, semicoke can
be used for power generation, employing the circulating fluidized-bed
boiler.

5. Oil shale ash can be utilized for the production of cement and
other building materials.

doi: 10.3176/oil.2012.4.07

Acknowledgements

We acknowledge the financial support from the Taishan Mountain
Scholar Constructive Engineering Foundation of Shandong province, China
(No.ts20120518), and the Science Foundation of China University of
Petroleum, Beijing, China (No.KYJJ2012-06-32).